Clamp for a hybrid switch

ABSTRACT

A switch having a drain, a source, and a control. The switch comprising a depletion-mode transistor including a first, a second, and a control terminal and an enhancement-mode transistor including a first, a second, and a control terminal. The first terminal of the depletion-mode transistor is the drain of the switch and the control of the depletion-mode transistor is coupled to the source of the switch. The control of the enhancement-mode transistor is coupled to the control of the switch, the second terminal of the enhancement-mode transistor is the source of the switch. The switch comprises a clamp circuit to clamp a voltage of the first terminal of the enhancement-mode transistor to a threshold, the clamp circuit comprises a resistor and a pn-junction device coupled between the first and second terminals of the enhancement-mode transistor and between the second terminal and the control of the depletion-mode transistor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/564,850, filed on Sep. 9, 2019, now pending, which is a continuationof U.S. patent application Ser. No. 16/220,684, filed on Dec. 14, 2018,now U.S. Pat. No. 10,461,740, which issued on Oct. 29, 2019, which is acontinuation of U.S. patent application Ser. No. 15/977,689, filed onMay 11, 2018, now U.S. Pat. No. 10,187,054, which issued on Jan. 22,2019, which is a continuation of U.S. patent application Ser. No.15/838,171, filed on Dec. 11, 2017, now U.S. Pat. No. 9,998,115, whichissued on Jun. 12, 2018, which is a divisional of U.S. patentapplication Ser. No. 15/246,395, filed Aug. 24, 2016, now U.S. Pat. No.9,871,510, which issued on Jan. 16, 2018. U.S. patent application Ser.No. 15/564,850, U.S. patent application Ser. No. 16/220,684, U.S. patentapplication Ser. No. 15/977,689, U.S. patent application Ser. No.15/838,171, and U.S. patent application Ser. No. 15/246,395 are herebyincorporated by reference in their entirety.

BACKGROUND INFORMATION Field of the Disclosure

The present invention relates generally to semiconductor devices andmore specifically to switches including a normally-off device and anormally-on device in a cascode configuration.

Background

Electronic devices use power to operate. Switched mode power convertersare commonly used due to their high efficiency, small size, and lowweight to power may of today's electronics. Conventional wall socketsprovide a high voltage alternating current. In a switching powerconverter, the high voltage alternating current (ac) input is convertedto provide a well-regulated direct current (dc) output through an energytransfer element. The switched mode power converter usually providesoutput regulation by sensing one or more inputs representative of one ormore output quantities and controlling the output in a closed loop. Inoperation, a switch is utilized to provide the desired output by varyingthe duty cycle (typically the ratio of the on time of the switch to thetotal switching period), varying the switching frequency, or varying thenumber of pulses per unit time of the switch in a switched mode powerconverter.

Various semiconductor devices may be used for the switch of the switchedmode power converter, such as a metal-oxide-semiconductor field-effecttransistor (MOSFET), insulated-gate bipolar transistor (IGBT), or abipolar junction transistor (BJT). These transistors may be fabricatedusing silicon (Si), silicon carbide (SiC), or gallium nitride (GaN)technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 illustrates a schematic of a switch including a leakage clampcircuit in accordance with the teachings of the present invention.

FIG. 2 illustrates one example of the leakage clamp circuit of FIG. 1 inaccordance with the teachings of the present invention.

FIG. 3A illustrates another example of the leakage clamp circuit of FIG.1 in accordance with the teachings of the present invention.

FIG. 3B illustrates a further example of the leakage clamp circuit ofFIG. 1 in accordance with the teachings of the present invention.

FIG. 4A illustrates an example cross-sectional view of a Zener diode ofthe leakage clamp circuit of FIG. 3B in accordance with the teachings ofthe present invention.

FIG. 4B illustrates another example cross-sectional view of a Zenerdiode of the leakage clamp circuit of FIG. 3B in accordance with theteachings of the present invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. Also, common but well-understoodelements that are useful or necessary in a commercially feasibleembodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. In other instances, well-known materials or methods have notbeen described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. Particular features, structures or characteristics may beincluded in an integrated circuit, an electronic circuit, acombinational logic circuit, or other suitable components that providethe described functionality. In addition, it is appreciated that thefigures provided herewith are for explanation purposes to personsordinarily skilled in the art and that the drawings are not necessarilydrawn to scale.

A cascode switch (or a hybrid switch) may include a normally on-deviceand a normally-off device. The cascode switch has three externalterminals, a source, a gate, and a drain. In one example, thenormally-on device may be a high-voltage GaN transistor, while thenormally-off device may be a low-voltage MOSFET. The source and gate ofthe normally-off device (e.g., MOSFET) are used as the source and gateof the cascode switch, while the drain of the normally-on device (e.g.,GaN transistor) is used as the drain of the cascode switch. The sourceof the normally-on device (e.g., GaN transistor) is coupled to the drainof the normally-off device (e.g., MOSFET). The normally-off device(e.g., MOSFET) is generally used to turn on and off the normally-ondevice (e.g., GaN transistor). A switch that is off (or open) cannotconduct current, while a switch that is on (or closed) may conductcurrent.

During operation, the normally-on device or the normally-off device mayhave leakage current. If the leakage current of the normally-off device(e.g., MOSFET) is greater than the leakage current of the normally-ondevice (e.g., GaN transistor), then the voltage at the intermediateterminal between the two devices substantially stays near ground.However, if the leakage current of the normally-on device (GaNtransistor) is greater than the leakage current of the normally-offdevice (MOSFET), then the voltage at the terminal between the twodevices may increase, which may potentially damage the cascode switch. Aleakage clamp may be used to prevent the voltage at the intermediateterminal between the two devices from increasing to an unsafe level (avoltage above a threshold voltage level). Further, the leakage clampclamps the voltage between the source terminal and the control terminalof the normally-on device at the threshold voltage level. This keeps thenormally-on device off when the normally-on device and the normally-offdevice are off, and prevents the voltage at the terminal between the twodevices from increasing to an unsafe level beyond the threshold voltagelevel.

FIG. 1 illustrates an example hybrid switch 100 including a normally-ondevice Q1 102 and a normally-off device Q2 104 coupled together in acascode configuration in accordance with the teachings of the presentinvention. The normally-on device Q1 102 is shown as an n-channeljunction field-effect transistor (JFET). Further, the normally-on deviceQ1 102 may be a high-voltage GaN transistor. The normally-on device Q1102 may also be referred to as a depletion-mode transistor. Thenormally-off device Q2 104 is shown as an n-channel MOSFET and may alsobe referred to as an enhancement-mode transistor. Further, thenormally-on device Q1 102 may be a high-voltage transistor (e.g., 100Vor greater), while the normally-off device Q2 104 may be a low-voltagetransistor (e.g., 100V or less).

The hybrid switch 100 includes three terminals, a drain terminal 106, agate terminal 108, and a source terminal 110. The gate and sourceterminals of the normally-off device Q2 104 are the gate terminal 108and source terminal 110, respectively, of the hybrid switch 100. Thedrain of the normally-on device Q1 102 is the drain terminal of thehybrid switch 100. Further, the source of the normally-on device Q1 102is coupled to the drain of the normally-off device Q2 104 at anintermediate node A 114 between the normally-on device Q1 102 and thenormally-off device Q2 104. As shown, the gate of the normally-on deviceQ1 102 is also coupled to the source terminal 110. Although the hybridswitch 100 illustrated in FIG. 1 shows no intervening elements betweenthe gate of the normally-on device Q1 102 and the source terminal 110,it is noted that intervening elements may be present (such as aresistor).

The hybrid switch 100 also includes a leakage clamp 112 coupled acrossthe normally-off device Q2 104. One end of the leakage clamp 112 iscoupled to the intermediate node A 114, while the other end is coupledto the source terminal S 110. In addition, the leakage clamp 112 iscoupled between the source and gate terminals of the normally-on deviceQ1 102 to clamp the voltage between the source and gate terminals of thenormally-on device Q1 102. The leakage clamp 112 may include variouscomponents, such as resistors, diodes, and transistors. Further, theleakage clamp 112 may be integrated with the normally-off device Q2 104.

In operation, the normally-off device Q2 104 is used to control theturning on and off of the normally-on device Q1 102. The leakage clamp112 may also be referred to as a soft clamp, as the primary use for theleakage clamp 112 is for low currents, such as leakage currents. Theleakage clamp 112 clamps the voltage at the intermediate node A 114caused by the leakage currents of both the normally-on device Q1 102 andthe normally-off device Q2 104. By clamping the voltage at theintermediate node A 114, the leakage clamp 112 may also help to avoid anexcessive source to gate voltage exerted on the normally-on device A1102 in the case of an imbalance of capacitance between the normally-ondevice Q1 102 and the normally-off device Q2 104. The leakage clamp 112clamps the voltage at the intermediate node A to a voltage level thatwould not raise reliability concerns for the gate of the normally-ondevice Q1 102. For instance, in one example the leakage clamp 112 clampsthe voltage at the intermediate node A to 24 volts (V).

FIG. 2 illustrates an example leakage clamp 212 included in a hybridswitch 200 in accordance with the teachings of the present invention. Itis appreciated that the hybrid switch 200 of FIG. 2 may be one exampleof the hybrid switch 100 of FIG. 1, and that similarly named andnumbered elements are therefore coupled and function similarly asdescribed above. As shown in the example depicted in FIG. 2, the leakageclamp 212 is coupled across the normally-off device Q2 204, and includestransistors 216 and 218 and resistor 220. Transistors 216 and 218 areshown as pnp bipolar transistors in FIG. 2. However, it is appreciatedthat in other examples, npn bipolar transistors may also be used. Theemitter terminals of transistors 216 and 218 are coupled to intermediatenode A 214, while the base terminals of transistors 216 and 218 arecoupled to each other. Further, the collector terminal of transistor 216is coupled to source terminal 210, while the collector terminal oftransistor 218 is coupled to its base terminal. One end of resistor 220is coupled to the collector terminal of transistor 218, while the otherend of resistor 220 is coupled to source terminal 210. In one example,transistor 218 is a lateral pnp bipolar transistor disposed in asemiconductor substrate, and transistor 216 may be a parasitic verticalpnp bipolar transistor disposed in the semiconductor substrate. In thisexample, transistor 216 may have its collector terminal connected to thesemiconductor substrate (in which transistor 216 is disposed), and itsbase terminal intrinsically part of the base of transistor 218. In otherwords, transistor 216 may be a parasitic transistor that exists as aresult of the construction of transistor 218.

In operation, when the voltage at intermediate node A 214 is low,transistors 216 and 218 are off, and current does not generally flowthrough leakage clamp 212. When the voltage at intermediate node A 214rises to a threshold voltage level, transistors 216 and 218 break downin a controlled manner, which clamps the voltage at intermediate node A214. In particular, when the voltage at intermediate node A 214 reachesthe threshold voltage level, transistor 218 turns on allowing current toflow (from intermediate node A 214, through transistor 218, to thesource terminal 210), while resistor 220 limits the amount of currentthat flows from the collector of transistor 218 to the source terminal210. During operation, the voltage at intermediate node A 214 may fallbelow ground, and substrate current may occur—in which current is pulledfrom the substrate (ground terminal). In general, substrate current isunwanted (and should be prevented) since the substrate current may go toother circuits coupled to the hybrid switch 200. The leakage clamp 212may reduce the likelihood of substrate current.

FIG. 3A illustrates another example of a leakage clamp 312 included in ahybrid switch 300 in accordance with the teachings of the presentinvention. It is appreciated that the hybrid switch 300 of FIG. 3A maybe another example of the hybrid switch 100 of FIG. 1, and thatsimilarly named and numbered elements are therefore coupled and functionsimilarly as described above. As shown in the example depicted in FIG.3A, the leakage clamp 312 is coupled across the normally-off device Q2304, and includes a Zener diode 322 and a resistor 324 coupled acrossnormally-off device Q2 304. In the example depicted in FIG. 3A, thecathode end of the Zener diode 322 is directly coupled to intermediatenode A 314, while the anode end is directly coupled to the resistor 324.The other end of the resistor 324 is directly coupled to the sourceterminal 310 of the hybrid switch 300. Although a Zener diode isillustrated in FIG. 3A, it is appreciated that in other examples, othertypes of diodes may be used instead.

In operation, the voltage at intermediate node A 314 is clamped by theZener diode 322. If the voltage at intermediate node A 314 exceeds athreshold voltage level, such as the breakdown voltage of the Zenerdiode 322, current conducts through Zener diode 322 (from intermediatenode A 314 to source terminal 310), and the resistor 324 limits theamount of current.

FIG. 3B illustrates another example of the leakage clamp 313 included ina hybrid switch 301 in accordance with the teachings of the presentinvention. It is appreciated that the hybrid switch 301 of FIG. 3B maybe yet another example of the hybrid switch 100 of FIG. 1, and thatsimilarly named and numbered elements are therefore coupled and functionsimilarly as described above. It is also noted that the hybrid switch301 of FIG. 3B is also similar to the hybrid switch 300 shown in FIG.3A, with one difference being the relative positioning of the resistor325 and Zener diode 323 in the leakage clamp 313. In particular, asshown in the example depicted FIG. 3B, the anode of the Zener diode 323is directly coupled to the source terminal 310, and the cathode of theZener diode 323 is directly coupled to one end of resistor 325. Theother end of resistor 325 is directly coupled to intermediate node A 314in the depicted example.

In operation, the voltage at intermediate node A 314 is clamped by theZener diode 323. If the voltage at intermediate node A 314 exceeds thebreakdown voltage of the Zener diode 323, current conducts through Zenerdiode 323 (from node A 314 to source terminal 310), and the resistor 325limits the amount of current. Since the anode of the Zener diode 323 isdirectly coupled to the source terminal 310, the construction of theanode terminal of the Zener diode 323 in the semiconductor material ofthe chip does not have to be isolated from the semiconductor substrate.In other words, the anode terminal of Zener diode 323 is a non-isolatedterminal from the semiconductor substrate.

For instance, in an example in which the non-isolated anode terminal ofZener diode 323 is a p-type anode and the semiconductor substrate inwhich the Zener diode 323 is disposed is a p-substrate, an n-typeisolation layer may be removed under the p-type anode junction. Theabsence of an isolation layer having the opposite polarity (i.e., theabsence of the n-type isolation layer to isolate the p-type anode ofZener diode 323 from the p-substrate) eliminates a parasitic npnjunction, which significantly reduces or eliminates parasitic npn gainin the device. In addition, the elimination of the parasitic npnjunction in Zener diode 323 allows the leakage clamp 313 to handle muchhigher current and be less susceptible to snapback in accordance withthe teachings of the present invention.

While the voltage at intermediate node A 314 may still fall below groundduring switching, as would be the case for example during constantcurrent mode zero voltage switching operation, Zener diode 323 would beforward biased and substrate current may still be possible. However,with the anode of the Zener diode 323 being a non-isolated terminal fromthe substrate, and with resistor 325 coupled between intermediate node A314 and the cathode of Zener diode 323, the cathode of Zener diode 323would experience less forward voltage due to the voltage drop throughresistor 325. As such, the substrate current through Zener diode 323 isless than the substrate current Zener diodes in other examples.Accordingly, the Zener diode 323 is more immune to substrate currentinjection current compared to other examples in accordance with theteachings of the present invention.

FIG. 4A illustrates an example cross section of a Zener diode 423Adisposed in semiconductor material, which is one example of Zener diode323 illustrated in FIG. 3B. Accordingly, similarly named and numberedelements are therefore coupled and function similarly as describedabove. As shown, the construction of the Zener diode 423A is notisolated from the substrate 430. By not isolating the Zener diode 423Afrom the substrate 430, snapback may be reduced. In the example, thep-substrate 430 is referenced to ground. A p doped semiconductor region,labeled PTOP layer 432, is disposed in the p-type substrate 430. In oneexample, the PTOP layer 432 is a region doped with p-type dopants, andhaving an average doping concentration of about 1e17. Further, ap-buried layer 434 is also disposed in the substrate 430 below the PTOPlayer 432. The p-buried layer 434 reduces parasitic npn gain by loweringthe base resistance. Although shown, the p-buried layer 434 may beoptional. Disposed in the PTOP layer 432 is an n+ doped cathode region436 and a p+ doped anode region 438, which provide contact to the PTOPlayer 432. As shown in the example depicted in FIG. 4A, p-buried layer434 is directly below, and vertically aligned with, p+ doped anoderegion 438. Further, the lateral bounds of p-buried layer 434 are largerthan the lateral bounds of p+ doped anode region 438. In one example,the n+ doped cathode region 436A may have doping concentration of about1e19. The p-n junction of Zener diode 423A shown in the example of FIG.4A is formed at the interface of the PTOP layer 432 and the n+ dopedcathode region 436.

Also disposed in the substrate 430 is an n-type guard ring 439, whichlaterally surrounds the Zener diode 423A. The guard ring 439 providesefficient collection of minority carriers in the substrate duringforward injection events, and may be used to improve the reverserecovery of the Zener diode 423A. Disposed in the guard ring 439 is ann+ region 440, which provides contact to the n-type guard ring 439.Metal regions 444 and 446 are disposed atop the p+ doped anode region438 and the n+ region 440, respectively, to contact to their respectiveregions. Further, as shown, metal regions 444 and 446 form the anodeterminal 426. In other words, the n-type guard ring 439 is shorted tothe anode terminal 426 of the Zener diode 423A.

As discussed above, in the depicted example, the anode terminal 426 is anon-isolated terminal from the p-substrate 430. For instance, as shownin the depicted example, anode terminal 426 is coupled to p+ doped anoderegion 438 through metal region 444, and the semiconductor substrate inwhich the Zener diode 423A is disposed is a p-substrate 430. Since anoderegion 438 and substrate 430 have the same polarity dopants (p dopants)and since there is an absence of an n-type isolation layer (i.e.,opposite polarity isolation layer from p+ doped anode region 438 andp-substrate 430) between p+ doped anode region 438 and p-substrate 430,the anode terminal 426 is a non-isolated terminal from p-substrate 430.The absence of an isolation layer having the opposite polarity betweenthe anode terminal 426 and the substrate 430 eliminates a parasitic npnjunction, which significantly reduces or eliminates parasitic npn gainin the device. In addition, the elimination of the parasitic npnjunction in Zener diode 423A allows a leakage clamp including Zenerdiode 423A to handle much higher current and be less susceptible tosnapback in accordance with the teachings of the present invention.

In the example depicted in FIG. 4A, the doping profiles of n-type guardring 439 and PTOP layer 432 are in direct contact with one another andmay even overlap slightly. However, in another example, a space mayexist between n-type guard ring 439 and PTOP layer 432. The space mayhave the same doping profile as p-substrate 430. Further, it should benoted that the n-type guard ring 439 extends further into the substratethan PTOP layer 432 in the depicted example.

Metal region 442 is disposed atop the n+ doped cathode region 436, andprovides contact to the n+ doped cathode region 436. As shown, metalregion 442 is the cathode terminal 428 of the Zener diode 423A. Oxidelayer 448 is shown as disposed atop the PTOP layer 432 and the n-typeguard ring 439 between metal regions 442 and 444, and 444 and 446. Theoxide layer 448 may be used to protect the device.

FIG. 4B illustrates another example cross section of a Zener diode 423Bdisposed in semiconductor material, which is another example of Zenerdiode 323 illustrated in FIG. 3B. In addition, it is appreciated thatthe cross-section of Zener diode 423B of FIG. 4B shares similaritieswith the example cross-section of Zener diode 423A of FIG. 4A.Accordingly, similarly named and numbered elements are therefore coupledand function similarly as described above. One difference between Zenerdiode 423B of FIG. 4B and Zener diode 423A of FIG. 4A is that instead ofa PTOP layer 432 disposed in the p-substrate 430 as shown in FIG. 4A,Zener diode 423B of FIG. 4B includes a p doped semiconductor region,labeled p-field region 452, and an n doped semiconductor region, labeledlow-voltage nwell (LV-nwell) 450, disposed in the p-substrate 430. Inthe depicted example, the doping profiles of the p-field region 452 andLV-nwell 450 are in direct contact and may even overlap slightly. Ingeneral, the p-field region 452 is disposed deeper into the p-substrate430 than the PTOP layer 432 of FIG. 4A, and the p-field region 452 has avariable doping profile. In one example, the average dopingconcentration of p-field region 452 is substantially 1e17.

Further, p+ doped anode region 438 is disposed in the p-field region 452to provide contact to the p-field region 452, while the n+ doped cathoderegion 436 is disposed in the LV-nwell 450 to provide contact to theLV-nwell 450. In the example depicted in FIG. 4B, meeting point betweenthe p-field region 452 and LV-nwell 450 is roughly midway between the p+doped anode region 438 and the n+ doped cathode region 436. For theexample of FIG. 4B, the p-n junction of Zener diode 423B is formed atthe interface of the p-field region 452 and the LV-nwell 450. As shown,the Zener diode is not isolated from the p-substrate 430, allowing ann-type isolation layer to be removed under the p-type anode terminal426, thereby reducing parasitic npn gain in the device that allows theZener diode 423B to handle much higher current without snapback. In theexample, the p-substrate 430 is referenced to ground.

Also disposed in the substrate 430 is an n-type guard ring 439, whichlaterally surrounds the Zener diode 423B. The guard ring 439 providesefficient collection of minority carriers in the substrate duringforward injection events, and may be used to improve the reverserecovery of the Zener diode 423B. Disposed in the guard ring 439 is ann+ region 440, which provides contact to the n-type guard ring 439.Metal regions 444 and 446 are disposed atop the p+ doped anode region438 and the n+ region, respectively, and provide contact to theirrespective regions. Further, as shown, metal regions 444 and 446 formthe anode terminal 426. In other words, the n-type guard ring 439 isshorted to the anode terminal 426 of the Zener diode 423B.

Similar to Zener diode 423A discussed above, the anode terminal 426 ofZener diode 423B is also a non-isolated terminal from the p-substrate430. For instance, as shown in the example depicted in FIG. 4B, anodeterminal 426 is coupled to p+ doped anode region 438 through metalregion 444, and the semiconductor substrate in which the Zener diode423B is disposed is a p-substrate 430. Since anode region 438 andsubstrate 430 have the same polarity dopants (p dopants) and since thereis an absence of an n-type isolation layer (i.e., opposite polarityisolation layer from p+ doped anode region 438 and p-substrate 430)between p+ doped anode region 438 and p-substrate 430, the anodeterminal 426 is a non-isolated terminal from p-substrate 430. Theabsence of an isolation layer having the opposite polarity between theanode terminal 426 and the substrate 430 eliminates a parasitic npnjunction, which significantly reduces or eliminates parasitic npn gainin the device. In addition, the elimination of the parasitic npnjunction in Zener diode 423B allows a leakage clamp including Zenerdiode 423B to handle much higher current and be less susceptible tosnapback in accordance with the teachings of the present invention.

As shown in the depicted example, n-type guard ring 439 may be in directcontact with the p-field region 452. These two structures may contacteach other proximate to n+ region 440. It is also noted that in thedepicted example, the depth of the LV-nwell 450, p-field region 452, andguard ring 439 into the substrate is approximately equal.

Metal region 442 is disposed atop the n+ doped cathode region 436 andprovides contact to the n+ doped cathode region 436. As shown, metalregion 442 is the cathode terminal 428 of the Zener diode 423B. Oxidelayer 448 is shown as disposed atop the LV-nwell 450, p-field region452, and the n-type guard ring 439, between the metal regions 442 and444, and 444 and 446. The oxide layer 448 may be used to protect thedevice.

The above description of illustrated examples of the present invention,including what is described in the Abstract, are not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible without departing from the broader spirit and scope of thepresent invention. Indeed, it is appreciated that the specific examplevoltages, currents, frequencies, power range values, times, etc., areprovided for explanation purposes and that other values may also beemployed in other embodiments and examples in accordance with theteachings of the present invention.

What is claimed is:
 1. A switch having a drain terminal, a sourceterminal and a control terminal, the switch comprising: a depletion-modetransistor comprising a first terminal, a second terminal, and a controlterminal, wherein the first terminal of the depletion-mode transistor isa drain of the switch and the control terminal of the depletion-modetransistor is coupled to the source terminal of the switch; anenhancement-mode transistor comprising a first terminal, a secondterminal, and a control terminal, wherein the control terminal of theenhancement-mode transistor is coupled to the control terminal of theswitch, the second terminal of the enhancement-mode transistor is thesource terminal of the switch, and the first terminal of theenhancement-mode transistor is coupled to the second terminal of thedepletion-mode transistor; and a clamp circuit coupled across the firstand second terminals of the enhancement-mode transistor, the clampcircuit configured to clamp a voltage across the first and secondterminals of the enhancement-mode transistor to a threshold, wherein theclamp circuit comprises: a current limiting resistor.
 2. The switch ofclaim 1, wherein the current limiting resistor is coupled between thefirst and second terminals of the enhancement-mode transistor.
 3. Theswitch of claim 2, wherein the control terminal of the depletion-modetransistor is referenced to ground.
 4. The switch of claim 2, the clampcircuit further comprising a pn-junction device coupled to the currentlimiting resistor and the first terminal of the enhancement-modetransistor, wherein the pn-junction device provides a controlledbreakdown characteristic.
 5. The switch of claim 2, wherein thepn-junction device is a Zener diode.
 6. The switch of claim 5, wherein:a cathode terminal of the Zener diode is coupled to the second terminalof the depletion-mode transistor and the first terminal of theenhancement-mode transistor; an anode terminal of the Zener diode iscoupled to the current limiting resistor; and the current limitingresistor is coupled to the second terminal of the enhancement-modetransistor.
 7. The switch of claim 5, wherein: a cathode terminal of theZener diode is coupled to one end of the current limiting resistor andan other end of the current limiting resistor is coupled to the secondterminal of the depletion-mode transistor and the first terminal of theenhancement-mode transistor; and an anode terminal of the Zener diode iscoupled the second terminal of the enhancement-mode transistor and thecontrol terminal of the depletion-mode transistor.
 8. The switch ofclaim 7, wherein the anode terminal of the Zener diode is a non-isolatedterminal from a substrate in which the Zener diode is disposed.
 9. Theswitch of claim 8, wherein a doped anode region coupled to the anodeterminal of the Zener diode and the substrate have same polaritydopants, wherein there is an absence of an isolation layer havingopposite polarity dopants between the doped anode region of the Zenerdiode and the substrate, and wherein the substrate is referenced toground.
 10. The switch of claim 2, wherein the clamp circuit furthercomprises a Zener diode coupled to the current limiting resistor and thefirst terminal of the enhancement-mode transistor, the Zener diodecomprising, a semiconductor substrate referenced to ground; a dopedanode region having a first polarity disposed above the semiconductorsubstrate, wherein the doped anode region is non-isolated from thesemiconductor substrate; a first doped semiconductor region having thefirst polarity disposed in the semiconductor substrate, wherein thedoped anode region is disposed in the first doped semiconductor region;and a doped cathode region having a second polarity.
 11. The switch ofclaim 10, wherein the Zener diode further comprises a second dopedsemiconductor region having the second polarity disposed in thesemiconductor substrate, wherein the doped cathode region is disposed inthe second doped semiconductor region, and wherein the p-n junction ofthe Zener diode is at an interface between the first doped semiconductorregion and the second doped semiconductor region.
 12. The switch ofclaim 2, the clamp circuit further comprises: a first transistor coupledbetween the first terminal of the enhancement-mode transistor and thecurrent limiting resistor; and a second transistor coupled between thefirst and second terminals of the enhancement-mode transistor, wherein acontrol terminal of the first transistor is coupled to a controlterminal of the second transistor and the current limiting resistor. 13.The switch of claim 12, wherein the first transistor comprises a lateralpnp bipolar transistor disposed in a substrate, wherein the secondtransistor includes a parasitic vertical pnp bipolar transistor disposedin the substrate, wherein the control terminal of the second transistoris coupled to the substrate, and wherein a base terminal of the secondtransistor is part of a base terminal of the first transistor.